Coatings based on epoxy resins are important industrial products. The largest volume of these products is used for the protection and decoration of large metal or concrete structures such as bridges, ships, industrial tanks, etc., where application of the coating must be performed under ambient conditions. Epoxy coatings of this type have proven themselves to offer an excellent combination of corrosion resistance, water resistance, abrasion resistance, solvent resistance and other desirable coatings properties, and do so in a cost effective manner.
Most epoxy resin coatings designed for ambient application employ polyfunctional amines as the curing agent, either alone or in some cases in combination with other curing agents. Several classes of amine curing agents are used commercially, including aliphatic amines, amidoamines, amine adducts, Mannich bases, and polyamides. They are described more fully in W. R. Ashcroft, Curing Agents for Epoxy Resins, in B. Ellis (ed.), Chemistry and Technology of Epoxy Resins, Blackie Academic and Professional, London, 1993, pp.37-71.
Among these curing agents, polyamides are a particularly important class of curing agent for the formulation of coatings. Polyamides comprise the reaction products of dimerized fatty acid (dimer acid) and polyethyleneamines, and usually but optionally, a monomeric fatty acid. Dimer acid is prepared by the oligomerization of certain monomeric fatty acids, usually tall oil fatty acid (TOFA), though sometimes other vegetable acids are substituted. Commercial products generally consist of mostly (&gt;70%) dimeric species, with the rest consisting mostly of trimers and higher oligomers, along with small amounts (generally less than 5%) of monomeric fatty acids. Any of the higher polyethyleneamines can be employed in the preparation of polyamides, such as diethylenetriamine (DETA), triethylenetetramine (TETA), tetraethylenepentamine (TEPA), or pentaethylenehexamine (PEHA), though in actual commercial practice the polyethyleneamine most commonly employed is TETA.
Polyamides are employed because they allow for the formulation of coatings with an excellent combination of water and corrosion resistance, most likely due to the hydrophobicity imparted by the fatty nature of the starting materials. They also can offer excellent flexibility, reasonable cure speeds (drying times), and less of a tendency to exude to the surface to cause surface appearance problems (known in the industry as exudate, blush and bloom) than some of the other hardener classes. In addition, due to the relatively low cost of fatty acids and dimer acid, polyamides are among the most cost effective of curing agents available.
Nevertheless, there are several properties of polyamide curing agents that would benefit from improvement.
As a result of environmental regulations, and also as the need to reduce solvent levels in coatings has been perceived by coatings manufacturers and their customers, there has been a need to reduce the viscosity of the binders employed in coatings, and epoxy based coatings are no exception. Polyamide curing agents have for some time been supplied in several grades of differing viscosity. Thus, one manufacturer (Air Products and Chemicals, Inc.) offers polyamide curing agents with a viscosity of about 330,000 cP at room temperature (Ancamide.RTM. 220), 40,000 cP (Ancamide 260A), and 11,000 cP (Ancamide 350A). However, as the viscosity of the curing agent is reduced in these products, it is generally found the amine hydrogen equivalent weight (AHEW) also decreases. For the polyamides described above, the AHEWs are 185, 120, and 100 respectively.
Epoxy resins are also available in many viscosities. The most commonly employed epoxy resins are those based upon the diglycidyl ether of bisphenol-A (DGEBA), and higher molecular weight oligomers prepared by the advancement of DGEBA with additional bisphenol-A. Such epoxy resins are generally difunctional or slightly less than difunctional, and characterized by their epoxy equivalent weight (EEW). Thus, bisphenol-A derived epoxy resin with an equivalent weight of 180 has a viscosity of about 8500 cP. Slightly increasing the EEW to 190 increases the viscosity to about 12,000 cP. At an equivalent weight of about 300 or so epoxy resins partially crystallize at a fairly rapid rate to a semi-solid, and above an equivalent weight of about 400 they are solids, and thus their viscosities cannot be measured at room temperature.
However, extrapolations of solvent cut samples of epoxy resins with EEWs in the range of 450 to 500 to 100% solids content suggest that the viscosity is at least many millions of cP at room temperature.
In the formulation of coatings, it is frequently advantageous to employ higher molecular weight epoxy resins, such as those with an equivalent weight of 450 to 500 (known in the industry as `1 type` resins). High molecular weight resins dramatically decrease the dry-to-touch time of the coating. Furthermore, higher molecular weight epoxy resins yield more flexible and impact resistant coatings than do lower molecular weight epoxy resins. Unfortunately, the high viscosity of the higher molecular weight epoxy resins requires the use of high levels of solvent in order to achieve a suitable application viscosity.
An approach to reducing the amount of solvent required in a coating formulation is to employ hardeners with reduced viscosity. However, as shown above, polyamide curing agents with lower viscosities also have lower equivalent weights. Normally, epoxy resins are combined with hardeners at stoichiometries of about 1:1 epoxy groups per amine hydrogen. At this ratio, most properties of the film, such as tensile strength, crosslink density, solvent resistance, etc., tend to be optimized. Consider 1:1 stoichiometric formulations based on 500 parts of a 500 EEW epoxy resin with a viscosity of many millions cP (on a solids basis), and the commercial polyamide curing agents described above. The formulation with Ancamide 220 curing agent contains 185 parts of a hardener with a 330,000 cP viscosity. The Ancamide 220 curing agent cuts the viscosity to some intermediate level. In formulation with Ancamide 350A curing agent, the curing agent has a viscosity of only 11,000 cP, but only 100 parts are used to dilute the epoxy resin. In practice, the final viscosities of these formulations are not dramatically different, and thus there is only a modest decrease in the amount of solvent required in the coating. Clearly, there is a need for low viscosity polyamide curing agents with higher equivalent weights than polyamides of the current art.
In many cases, restrictions on the use of solvents require that low molecular weight epoxy resins be used in place of the preferred, higher molecular weight epoxy resins irrespective of the viscosity curing agent employed. As mentioned above, this increases the dry-to-touch time of the coating. Thus, there is a need for curing agents that reduce dry-to-touch times of epoxy coatings, particularly those based on liquid epoxy resins.
As mentioned above, there is a tendency for curing agents to rise to the surface of a coating during the cure. This can leave a greasy film on the surface of the coating which detracts from the appearance and which can also lead to intercoat adhesion failure if the epoxy is a primer or mid-coat. Under adverse application conditions such as high humidity, concentration of the amines at the surface can result in the formulation of whitish precipitates on the surface which are probably bicarbonate and/or carbamate salts, a problem known in the industry as blush. While polyamides are better than certain classes of curing agents, particularly amidoamines and unmodified polyethyleneamines, in this regard, they are still far from perfect. In addition, the high viscosity polyamides of the current art tend to exhibit less exudate and blush than the lower viscosity polyamides. It is found that by waiting a period of time, generally 0.5 to 1 hour or more after mixing the epoxy and amine components of the formulation, exudate and blush can be reduced or even eliminated. This is known as an induction time.
As solvent levels and epoxy molecular weight have been reduced in epoxy coating formulations, however, it has been found that pot lives have also been reduced. The pot life is the time available after mixing the amine and epoxy components of the formulation during which the viscosity remains low enough to allow application. The decrease in pot life is the result of simple chemical kinetics: reduction of solvent content and equivalent weight both result in an increase in the concentration of functional groups, and hence an increase in the rate of reactions that lead to increased viscosity. Thus, there is a need for curing agents with decreased exudate and blush, so that induction times can be reduced or eliminated.
For good protection of metallic substrates, it is necessary that the coating maintain good adhesion to the substrate, particularly under wet conditions such as the Cleveland condensing humidity test. While epoxy coatings generally have good adhesion, there is still a need for improved adhesion, particularly over poor substrates such as cold-rolled steel (CRS).
Finally, there is a need for curing agents that can lead to epoxy coating compositions with greater corrosion resistance, leading to coatings with longer life in service.
U.S. Pat. No. 2,450,940 and U.S. Pat. No. 2,705,223 both describe the preparation of polyamide resins useful for curing epoxy resins by the condensation of dimerized or polymerized fatty acids with polyethyleneamines such as ethylenediamine (EDA) and DETA.
U.S. Pat. No. 5,021,482 describes the preparation of polyamides from polymerized fatty acid and a mixture of amines comprising a polyalkylene polyamine and an N-aminoalkyl-piperazine, preferably N-aminoethylpiperazine (AEP). The polyamides are utilized as adhesion promoters for PVC plastisols. Because high amine content in such an adhesion promoter destroys the acid catalysts employed in top coats applied to such plastisols, this invention is directed toward the preparation of polyamides with an amine value less than about 225. In order to achieve these low amine values, the percent by weight of amines utilized is less than 30%, preferably less than 25%. Although no viscosity of the neat polyamides prepared in '482 is reported, the viscosity of the product of example 1 is 8,880 cP at 25.degree. C., at a calculated solids of only 50%, assuming that 1 mole of water is lost for every 295 g of polymerized fatty acid in the composition. Thus, these products are very high in viscosity, and of little value in modern coatings applications, where environmental regulations require that only limited amounts of solvent can be utilized in the final coating formulation.
CS 266519 discloses an extremely broad range of polyamide resins prepared by condensing carboxylic acids (av. mol. wt. 146-650) with polyamines composed of 20-90% aliphatic polyamines H.sub.2 N(CH.sub.2 CH.sub.2 NHCH.sub.2 CH.sub.2).sub.1-4 NH.sub.2 and 10-80% heterocyclic polyamines H.sub.2 N(CH.sub.2 CH.sub.2 NH).sub.0-4 CH.sub.2 CH.sub.2 Z(CH.sub.2 CH.sub.2 N).sub.0-4 H.